## Notes for Quantum Field Theory I (phy2403) now available in paper on amazon

My notes (423 pages, 6″x9″) from the fall 2018 session of the University of Toronto Quantum Field Theory I course (PHY2403), taught by Prof. Erich Poppitz, are now available on amazon.com (through kindle-direct-publishing, formerly createspace).

These notes are available in three forms, two free, and one paper:

• On amazon (kindle-direct-publishing) for \$11 USD,
• As a free PDF,
• As latex sources (, makefiles, figures, …) to build/modify yourself.

This book is dedicated to dad.

### Warning to students

These notes are no longer redacted and include whatever portions of the problem set 1-4 solutions I completed, errors and all.  In the event that any of the problem sets are recycled for future iterations of the course, students who are taking the course (all mature grad students pursuing science for the love of it, not for grades) are expected to act responsibly, and produce their own solutions, within the constraints provided by the professor.

### Topics

The official course outline included:

1. Introduction: Energy and distance scales; units and conventions. Uncertainty relations in the relativistic domain and the need for multiple particle description.
2. Canonical quantization. Free scalar field theory.
3. Symmetries and conservation laws.
4. Interacting fields: Feynman diagrams and the S matrix; decay widths and phase space.
5. Spin 1/2 fields: Spinor representations, Dirac and Weyl spinors, Dirac equation. Quantizing fermi fields and statistics.
6. Vector fields and Quantum electrodynamics.

## Spinor solutions with alternate $$\gamma^0$$ representation.

January 2, 2019 phy2403 , ,

This follows an interesting derivation of the $$u, v$$ spinors [2], adding some details.

In class (QFT I) and [3] we used a non-diagonal $$\gamma^0$$ representation
\label{eqn:spinorSolutions:20}
\gamma^0 =
\begin{bmatrix}
0 & 1 \\
1 & 0
\end{bmatrix},

whereas in [2] a diagonal representation is used
\label{eqn:spinorSolutions:40}
\gamma^0 =
\begin{bmatrix}
1 & 0 \\
0 & -1
\end{bmatrix}.

This representation makes it particularly simple to determine the form of the $$u, v$$ spinors. We seek solutions of the Dirac equation
\label{eqn:spinorSolutions:60}
\begin{aligned}
0 &= \lr{ i \gamma^\mu \partial_\mu – m } u(p) e^{-i p \cdot x} \\
0 &= \lr{ i \gamma^\mu \partial_\mu – m } v(p) e^{i p \cdot x},
\end{aligned}

or
\label{eqn:spinorSolutions:80}
\begin{aligned}
0 &= \lr{ \gamma^\mu p_\mu – m } u(p) e^{-i p \cdot x} \\
0 &= -\lr{ \gamma^\mu p_\mu + m } v(p) e^{i p \cdot x}.
\end{aligned}

In the rest frame where $$\gamma^\mu p_\mu = E \gamma^0$$, where $$E = m = \omega_\Bp$$, these take the particularly simple form
\label{eqn:spinorSolutions:100}
\begin{aligned}
0 &= \lr{ \gamma^0 – 1 } u(E, \Bzero) \\
0 &= \lr{ \gamma^0 + 1 } v(E, \Bzero).
\end{aligned}

This is a nice relation, as we can determine a portion of the structure of the rest frame $$u, v$$ that is independent of the Dirac matrix representation
\label{eqn:spinorSolutions:120}
\begin{aligned}
u(E, \Bzero) &= (\gamma^0 + 1) \psi \\
v(E, \Bzero) &= (\gamma^0 – 1) \psi
\end{aligned}

Similarly, and more generally, we have
\label{eqn:spinorSolutions:140}
\begin{aligned}
u(p) &= (\gamma^\mu p_\mu + m) \psi \\
v(p) &= (\gamma^\mu p_\mu – m) \psi
\end{aligned}

also independent of the representation of $$\gamma^\mu$$. Looking forward to non-matrix representations of the Dirac equation ([1]) note that we have not yet imposed a spinorial structure on the solution
\label{eqn:spinorSolutions:260}
\psi
=
\begin{bmatrix}
\phi \\
\chi
\end{bmatrix},

where $$\phi, \chi$$ are two component matrices.

The particular choice of the diagonal representation \ref{eqn:spinorSolutions:40} for $$\gamma^0$$ makes it simple to determine additional structure for $$u, v$$. Consider the rest frame first, where
\label{eqn:spinorSolutions:160}
\begin{aligned}
\gamma^0 – 1 &=
\begin{bmatrix}
1 & 0 \\
0 & -1
\end{bmatrix}

\begin{bmatrix}
1 & 0 \\
0 & -1
\end{bmatrix}
=
\begin{bmatrix}
0 & 0 \\
0 & 2
\end{bmatrix} \\
\gamma^0 + 1 &=
\begin{bmatrix}
1 & 0 \\
0 & -1
\end{bmatrix}
+
\begin{bmatrix}
1 & 0 \\
0 & -1
\end{bmatrix}
=
\begin{bmatrix}
2 & 0 \\
0 & 0
\end{bmatrix},
\end{aligned}

so we have
\label{eqn:spinorSolutions:280}
\begin{aligned}
u(E, \Bzero) &=
\begin{bmatrix}
2 & 0 \\
0 & 0
\end{bmatrix}
\begin{bmatrix}
\phi \\
\chi
\end{bmatrix} \\
v(E, \Bzero) &=
\begin{bmatrix}
0 & 0 \\
0 & 2
\end{bmatrix}
\begin{bmatrix}
\phi \\
\chi
\end{bmatrix}
\end{aligned}

Therefore a basis for the spinors $$u$$ (in the rest frame), is
\label{eqn:spinorSolutions:180}
u(E, \Bzero) \in \setlr{
\begin{bmatrix}
1 \\
0 \\
0 \\
0
\end{bmatrix},
\begin{bmatrix}
0 \\
1 \\
0 \\
0
\end{bmatrix}
},

and a basis for the rest frame spinors $$v$$ is
\label{eqn:spinorSolutions:200}
v(E, \Bzero) \in \setlr{
\begin{bmatrix}
0 \\
0 \\
1 \\
0
\end{bmatrix},
\begin{bmatrix}
0 \\
0 \\
0 \\
1
\end{bmatrix}
}.

Using the two spinor bases $$\zeta^a, \eta^a$$ notation from class, we can write these
\label{eqn:spinorSolutions:220}
\begin{aligned}
u^a(E, \Bzero) &=
\begin{bmatrix}
\zeta^a \\
0
\end{bmatrix},
v^a(E, \Bzero) &=
\begin{bmatrix}
0 \\
\eta^a \\
\end{bmatrix}.
\end{aligned}

For the non-rest frame solutions, [2] opts not to boost, as in [3], but to use the geometry of $$\gamma^\mu p_\mu \pm m$$. With their diagonal representation of $$\gamma^0$$ those are
\label{eqn:spinorSolutions:240}
\begin{aligned}
\gamma^\mu p_\mu – m
&=
p_0
\begin{bmatrix}
1 & 0 \\
0 & -1
\end{bmatrix}
+
p_k
\begin{bmatrix}
0 & \sigma^k \\
– \sigma^k & 0
\end{bmatrix}

m
\begin{bmatrix}
1 & 0 \\
0 & 1
\end{bmatrix}
=
\begin{bmatrix}
E – m & – \Bsigma \cdot \Bp \\
\Bsigma \cdot \Bp & -E – m
\end{bmatrix} \\
\gamma^\mu p_\mu + m
&=
p_0
\begin{bmatrix}
1 & 0 \\
0 & -1
\end{bmatrix}
+
p_k
\begin{bmatrix}
0 & \sigma^k \\
– \sigma^k & 0
\end{bmatrix}
+
m
\begin{bmatrix}
1 & 0 \\
0 & 1
\end{bmatrix}
=
\begin{bmatrix}
E + m & – \Bsigma \cdot \Bp \\
\Bsigma \cdot \Bp & -E + m
\end{bmatrix} \\
\end{aligned}

Let’s assume that the arbitrary momentum solutions \ref{eqn:spinorSolutions:140} are each proportional to the rest frame solutions
\label{eqn:spinorSolutions:300}
\begin{aligned}
u^a(p) &= (\gamma^\mu p_\mu + m) u^a(E, \Bzero) \\
v^a(p) &= (\gamma^\mu p_\mu – m) u^a(E, \Bzero).
\end{aligned}

Plugging in \ref{eqn:spinorSolutions:240} gives
\label{eqn:spinorSolutions:320}
\begin{aligned}
u^a(p) &=
\begin{bmatrix}
(E + m) \zeta^a \\
(\Bsigma \cdot \Bp ) \zeta^a
\end{bmatrix} \\
v^a(p) &=
\begin{bmatrix}
(\Bsigma \cdot \Bp) \eta^a \\
(E + m) \eta^a
\end{bmatrix},
\end{aligned}

where an overall sign on $$v^a(p)$$ has been dropped. Let’s check the assumption that the rest frame and general solutions are so simply related
\label{eqn:spinorSolutions:340}
\begin{aligned}
\lr{ \gamma^\mu p_\mu – m } u^a(p)
&=
\begin{bmatrix}
E – m & – \Bsigma \cdot \Bp \\
\Bsigma \cdot \Bp & -E – m
\end{bmatrix}
\begin{bmatrix}
(E + m) \zeta^a \\
(\Bsigma \cdot \Bp ) \zeta^a
\end{bmatrix} \\
&=
\begin{bmatrix}
(E^2 – m^2 – \Bp^2) \zeta^a \\
0
\end{bmatrix} \\
&= 0,
\end{aligned}

and
\label{eqn:spinorSolutions:360}
\begin{aligned}
\lr{ \gamma^\mu p_\mu + m } v^a(p)
&=
\begin{bmatrix}
E + m & – \Bsigma \cdot \Bp \\
\Bsigma \cdot \Bp & -E + m
\end{bmatrix}
\begin{bmatrix}
(\Bsigma \cdot \Bp ) \eta^a \\
(E + m) \eta^a \\
\end{bmatrix} \\
&=
\begin{bmatrix}
0 \\
\Bp^2 + m^2 – E^2
\end{bmatrix} \\
&= 0.
\end{aligned}

Everything works out nicely. The form of the solution for this representation of $$\gamma^0$$ is much simpler than the Chiral solution that we found in class. We end up with an explicit split of energy and spatial momentum components in the spinor solutions, instead of factors involving $$p \cdot \sigma$$ and $$p \cdot \overline{\sigma}$$, which are arguably nicer from a Lorentz invariance point of view.

# References

[1] C. Doran and A.N. Lasenby. Geometric algebra for physicists. Cambridge University Press New York, Cambridge, UK, 1st edition, 2003.

[2] Claude Itzykson and Jean-Bernard Zuber. Quantum field theory. McGraw-Hill, 1980.

[3] Michael E Peskin and Daniel V Schroeder. An introduction to Quantum Field Theory. Westview, 1995.

## Final first draft of complete notes for UofT PHY2403, QFT I .

December 27, 2018 phy2403 , ,

I’ve now uploaded a new version of my class notes for PHY2403, the UofT Quantum Field Theory I course, taught this year by Prof. Erich Poppitz.

This update adds notes for all remaining lectures (up to and including lecture 23.)  I’ve made a pass with a spellchecker to correct some of the aggregious spelling erorss, and also redrawn three figures, replacing photos, which cuts the size in half!

I’ve posted the redacted version (316 pages).  The full version, with my problem set solutions (including errors) is 409 pages.

Feel free to contact me for the complete version (i.e. including my problem set solutions, with errors) of any of these notes, provided you are not asking because you are taking or planning to take this course.

Contents:

• Preface
• Contents
• List of Figures
• 1 Fields, units, and scales.
• 1.1 What is a field?
• 1.2 Scales.
• 1.2.2 Compton wavelength.
• 1.2.3 Relations.
• 1.3 Natural units.
• 1.4 Gravity.
• 1.5 Cross section.
• 1.6 Problems.
• 2 Lorentz transformations.
• 2.1 Lorentz transformations.
• 2.2 Determinant of Lorentz transformations.
• 2.3 Problems.
• 3 Classical field theory.
• 3.1 Field theory.
• 3.2 Actions.
• 3.3 Principles determining the form of the action.
• 3.4 Principles (cont.)
• 3.4.1 d = 2.
• 3.4.2 d = 3.
• 3.4.3 d = 4.
• 3.4.4 d = 5.
• 3.5 Least action principle.
• 3.6 Problems.
• 4 Canonical quantization, Klein-Gordon equation, SHOs, momentum space representation, raising and lowering operators.
• 4.1 Canonical quantization.
• 4.2 Canonical quantization (cont.)
• 4.3 Momentum space representation.
• 4.4 Quantization of Field Theory.
• 4.5 Free Hamiltonian.
• 4.6 QM SHO review.
• 4.7 Discussion.
• 4.8 Problems.
• 5 Symmetries.
• 5.1 Switching gears: Symmetries.
• 5.2 Symmetries.
• 5.3 Spacetime translation.
• 5.4 1st Noether theorem.
• 5.5 Unitary operators.
• 5.6 Continuous symmetries.
• 5.7 Classical scalar theory.
• 5.8 Last time.
• 5.9 Examples of symmetries.
• 5.10 Scale invariance.
• 5.11 Lorentz invariance.
• 5.12 Problems.
• 6 Lorentz boosts, generators, Lorentz invariance, microcausality.
• 6.1 Lorentz transform symmetries.
• 6.2 Transformation of momentum states.
• 6.3 Relativistic normalization.
• 6.4 Spacelike surfaces.
• 6.5 Condition on microcausality.
• 7 External sources.
• 7.1 Harmonic oscillator.
• 7.2 Field theory (where we are going).
• 7.3 Green’s functions for the forced Klein-Gordon equation.
• 7.4 Pole shifting.
• 7.5 Matrix element representation of the Wightman function.
• 7.6 Retarded Green’s function.
• 7.7 Review: “particle creation problem”.
• 7.8 Digression: coherent states.
• 7.9 Problems.
• 8 Perturbation theory.
• 8.1 Feynman’s Green’s function.
• 8.2 Interacting field theory: perturbation theory in QFT.
• 8.3 Perturbation theory, interaction representation and Dyson formula.
• 8.4 Next time.
• 8.5 Review.
• 8.6 Perturbation.
• 8.7 Review.
• 8.8 Unpacking it.
• 8.9 Calculating perturbation.
• 8.10 Wick contractions.
• 8.11 Simplest Feynman diagrams.
• 8.12 Phi fourth interaction.
• 8.13 Tree level diagrams.
• 8.14 Problems.
• 9 Scattering and decay.
• 9.2 Definitions and motivation.
• 9.3 Calculating interactions.
• 9.4 Example diagrams.
• 9.5 The recipe.
• 9.6 Back to our scalar theory.
• 9.7 Review: S-matrix.
• 9.8 Scattering in a scalar theory.
• 9.9 Decay rates.
• 9.10 Cross section.
• 9.11 More on cross section.
• 9.12 d(LIPS)_2.
• 9.13 Problems.
• 10 Fermions, and spinors.
• 10.1 Fermions: R3 rotations.
• 10.2 Lorentz group.
• 10.3 Weyl spinors.
• 10.4 Lorentz symmetry.
• 10.5 Dirac matrices.
• 10.6 Dirac Lagrangian.
• 10.7 Review.
• 10.8 Dirac equation.
• 10.9 Helicity.
• 10.10 Next time.
• 10.11 Review.
• 10.12 Normalization.
• 10.13 Other solution.
• 10.14 Lagrangian.
• 10.15 General solution and Hamiltonian.
• 10.16 Review.
• 10.17 Hamiltonian action on single particle states.
• 10.18 Spacetime translation symmetries.
• 10.19 Rotation symmetries: angular momentum operator.
• 10.20 U(1)_V symmetry: charge!
• 10.21 U(1)_A symmetry: what was the charge for this one called?
• 10.22 CPT symmetries.
• 10.23 Review.
• 10.24 Photon.
• 10.25 Propagator.
• 10.26 Feynman rules.
• 10.27 Example: muon pair production
• 10.28 Measurement of intermediate quark scattering processes.
• 10.29 Problems.
• A Useful formulas and review.
• A.1 Review of old material.
• A.2 Useful results from new material.
• B Momentum of scalar field.
• B.1 Expansion of the field momentum.
• B.2 Conservation of the field momentum.
• C Reflection using Pauli matrices.
• D Explicit expansion of the Dirac u,v spinors.
• D.1 Compact representation of
• E Mathematica notebooks
• Bibliography

## PHY2403H Quantum Field Theory. Lecture 23: QED and QCD interaction Lagrangian, Feynman propagator and rules for Fermions, hadron pair production, scattering cross section, quark pair production. Taught by Prof. Erich Poppitz

Here is a link to [a PDF with my notes for the final QFT I lecture.] That lecture followed [1] section 5.1 fairly closely (filling in some details, leaving out some others.)

This lecture

• Introduced an interaction Lagrangian with QED and QCD interaction terms
\begin{equation*}
\LL_{\text{QED}}
=
– \inv{4} F_{\mu\nu} F^{\mu\nu}
+
\overline{\Psi}_e \lr{ i \gamma^\mu \partial_\mu – m } \Psi_e

e \overline{\Psi}_e \gamma_\mu \Psi_e A^\mu
+
\overline{\Psi}_\mu \lr{ i \gamma^\mu \partial_\mu – m } \Psi_\mu

e \overline{\Psi}_\mu \gamma_\mu \Psi_\mu A^\mu,
\end{equation*}
as well as the quark interaction Lagrangian
\begin{equation*}
\LL_{\text{quarks}} = \sum_q \overline{\Psi}_q \lr{ i \gamma^\mu – m_q } \Psi_q + e Q_q \overline{\Psi}_q \gamma^\nu \Psi_q A_\nu.
\end{equation*}
• The Feynman propagator for Fermions was calculated
\begin{equation*}
\expectation{ T( \Psi_\alpha(x) \Psi_\beta(x) }_0
=
\lr{ \gamma^\mu_{\alpha\beta} \partial_\mu^{(x)} + m } D_F(x – y)
=
\int \frac{d^4 p}{(2 \pi)^4 } \frac{ i ( \gamma^\mu_{\alpha\beta} p_\mu + m ) }{p^2 – m^2 + i \epsilon} e^{-i p \cdot (x – y)}.
\end{equation*}
• We determined the Feynman rules for Fermion diagram nodes and edges.
The Feynman propagator for Fermions is
\begin{equation*}
\frac{ i \lr{ \gamma^\mu p_\mu + m } }{p^2 – m^2 + i \epsilon},
\end{equation*}
whereas the photon propagator is
\begin{equation*}
\expectation{ A_\mu A_\nu } = -i \frac{g_{\mu\nu}}{q^2 + i \epsilon}.
\end{equation*}
• Muon pair production

We then studied muon pair production in detail, and determined the form of the scattering matrix element
\begin{equation*}
i M
=
i \frac{e^2}{q^2}
\overline{v}^{s’}(p’) \gamma^\rho u^s(p)
\overline{u}^r(k) \gamma_\rho v^{r’}(k’),
\end{equation*}
where the $$(2 \pi)^4 \delta^4(…)$$ term hasn’t been made explicit, and detemined that the average of its square over all input and output polarization (spin) states was
\begin{equation*}
\inv{4} \sum_{ss’, rr’} \Abs{M}^2
=
\frac{e^4}{4 q^4}
\textrm{tr}{ \lr{
\lr{ \gamma^\alpha {k’}_\alpha – m_\mu }
\gamma_\nu
\lr{ \gamma^\beta {k}_\beta + m_\mu }
\gamma_\mu
}}
\times
\textrm{tr}{ \lr{
\lr{ \gamma^\kappa {p}_\kappa + m_e }
\gamma^\nu
\lr{ \gamma^\rho {p’}_\rho – m_e }
\gamma^\mu
}}.
\end{equation*}.
In the CM frame (neglecting the electron mass, which is small relative to the muon mass), this reduced to
\begin{equation*}
\inv{4} \sum_{\text{spins}} \Abs{M}^2
=
\frac{8 e^4}{q^4}
\lr{
p \cdot k’ p’ \cdot k
+ p \cdot k p’ \cdot k’
+ p \cdot p’ m_\mu^2
}.
\end{equation*}

• We computed the differential cross section
\begin{equation*}
{\frac{d\sigma}{d\Omega}}_{\text{CM}}
=
\frac{\alpha^2}{4 E_{\text{CM}}^2 }
\sqrt{ 1 – \frac{m_\mu^2}{E^2} }
\lr{
1 + \frac{m_\mu^2}{E^2}
+ \lr{ 1 – \frac{m_\mu^2}{E^2} } \cos^2\theta
},
\end{equation*}
and the total cross section
\begin{equation*}
\sigma_{\text{total}}
=
\frac{4 \pi \alpha^2}{3 E_{\text{CM}}^2 }
\sqrt{ 1 – \frac{m_\mu^2}{E^2} }
\lr{
1 + \inv{2} \frac{m_\mu^2}{E^2}
},
\end{equation*}
and compared that to the cross section that we was determined with the dimensional analysis handwaving at the start of the course.
• We finished off with a quick discussion of quark pair production, and how some of the calculations we performed for muon pair production can be used to measure and validate the intermediate quark states that were theorized as carriers of the strong force.

# References

[1] Michael E Peskin and Daniel V Schroeder. An introduction to Quantum Field Theory. Westview, 1995.

## Dirac spinor relations after rest frame boost

December 18, 2018 phy2403 , , , , ,

In [1], Prof Osmond explicitly boosts a $$u^s(p_0)$$ Dirac spinor from the rest frame with rest frame energy $$p_0$$.
After doing so he claims the identification
\label{eqn:squarerootpsigma:20}
\begin{aligned}
\sqrt{m} e^{-\inv{2} \eta \sigma^3} &= \sqrt{ p \cdot \sigma } \\
\sqrt{m} e^{\inv{2} \eta \sigma^3} &= \sqrt{ p \cdot \overline{\sigma} },
\end{aligned}

for the components of $$u^s(\Lambda p_0)$$.

Let’s verify this by squaring. First
\label{eqn:squarerootpsigma:40}
e^{\pm \inv{2} \eta \sigma^3 }
=
\cosh\lr{ \inv{2} \eta \sigma^3 }
\pm
\sinh\lr{ \inv{2} \eta \sigma^3 } \sigma^3,

which squares to (FIXME: link to uvspinor.nb)
\label{eqn:squarerootpsigma:60}
\lr{ e^{\pm \inv{2} \eta \sigma^3 } }^2
=
\begin{bmatrix}
e^{\pm \eta} & 0 \\
0 & e^{\mp \eta}
\end{bmatrix}.

Explicitly boosting the rest energy $$p_0$$ gives
\label{eqn:squarerootpsigma:80}
\begin{bmatrix}
p_0 \\
0
\end{bmatrix}
\rightarrow
\begin{bmatrix}
\cosh\eta & \sinh\eta \\
\sinh\eta & \cosh\eta \\
\end{bmatrix}
\begin{bmatrix}
p_0 \\
0
\end{bmatrix}
=
p_0
\begin{bmatrix}
\cosh\eta \\
\sinh\eta
\end{bmatrix},

so after the boost
\label{eqn:squarerootpsigma:100}
\begin{aligned}
p \cdot \sigma
&\rightarrow
p_0 \lr{ \cosh \eta – \sinh \eta \sigma^3 } \\
&= p_0
\begin{bmatrix}
\cosh\eta – \sinh\eta & 0 \\
0 & \cosh\eta + \sinh\eta
\end{bmatrix} \\
&=
p_0
\begin{bmatrix}
e^{-\eta} & 0 \\
0 & e^{\eta}
\end{bmatrix},
\end{aligned}

where $$p_0 = m$$ is still the rest frame energy. However, according to \ref{eqn:squarerootpsigma:60} this is exactly
\label{eqn:squarerootpsigma:120}
\lr{\sqrt{m} e^{-\inv{2} \eta \sigma^3 }}^2

Since $$p \cdot \overline{\sigma}$$ flips the signs of the spatial momentum, we have shown that
\label{eqn:squarerootpsigma:140}
\begin{aligned}
\lr{\sqrt{m} e^{-\inv{2} \eta \sigma^3 }}^2 &= p \cdot \sigma \\
\lr{\sqrt{m} e^{\inv{2} \eta \sigma^3 }}^2 &= p \cdot \overline{\sigma},
\end{aligned}

which isn’t a full proof of the claimed result (i.e. the most general orientation isn’t considered), but at least validates the claim.

# References

[1] Dr. Tobias Osborne. Qft lecture 15, dirac equation, boost from stationary frame. Youtube. URL https://youtu.be/J2lV8uNx0LU?list=PLDfPUNusx1EpRs-wku83aqYSKfR5fFmfS&t=4328. [Online; accessed 18-December-2018].